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Novel Details of Calsequestrin Gel Conformation in Situ *

S. Perni,Matthew Close,C. Franzini-armstrong

Published 2013 in Journal of Biological Chemistry

ABSTRACT

Background: Calsequestrin is essential to keep a high calcium concentration inside the sarcoplasmic reticulum of muscle fibers. Results: In situ, calsequestrin polymers appear to form a three-dimensional structure with repeated nodal points. Conclusion: A three-dimensional calsequestrin polymer matrix is very suitable for its spatially confined calcium storage function. Significance: The calsequestrin structure has been extensively studied in ex vivo systems. This approach illustrates the behavior of the protein while still in its physiological cell localization. Calsequestrin (CASQ) is the major component of the sarcoplasmic reticulum (SR) lumen in skeletal and cardiac muscles. This calcium-binding protein localizes to the junctional SR (jSR) cisternae, where it is responsible for the storage of large amounts of Ca2+, whereas it is usually absent, at least in its polymerized form, in the free SR. The retention of CASQ inside the jSR is due partly to its association with other jSR proteins, such as junctin and triadin, and partly to its ability to polymerize, in a high Ca2+ environment, into an intricate gel that holds the protein in place. In this work, we shed some light on the still poorly described in situ structure of polymerized CASQ using detailed EM images from thin sections, with and without tilting, and from deep-etched rotary-shadowed replicas. The latter directly illustrate the fundamental network nature of polymerized CASQ, revealing repeated nodal points connecting short segments of the linear polymer.

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